6.4 Eutrophication and Dead Zones

Notes

Eutrophication is an increase in nutrients (especially nitrogen and phosphorus) in a water body that causes excess plant and algae growth.

• Cultural eutrophication is the human-caused form, from fertilizer, sewage, and detergents.
• Sequence: nutrients enter, algal bloom, algae block sunlight and die, bacteria decompose the dead algae and use up dissolved oxygen, hypoxia (low oxygen) develops, and fish and other aerobic organisms die.
• Dead zones are oxygen-poor areas. A large one forms each summer in the Gulf of Mexico, linked to nutrient runoff carried by the Mississippi River.
• Prevention: reduce fertilizer use, plant buffer strips along streams, restore wetlands, upgrade sewage treatment, and ban phosphates in detergents.
• Example: after a lawn-fertilizer runoff event, a pond turns green with algae, and a week later fish float dead because oxygen is depleted by decomposers.